Dolin seeks help in identifying poor “reliability” in electric motors

Friday - 02/02/2007 11:06

Dolin is calling upon the electric motor industry to create greater transparency with its technical motor data by making available information that gives a clear indication of motor reliability, such as running temperature, alongside efficiency levels and motor noise.

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While Johnny, Dolin general manager for motors and drives in Vietnam, acknowledges that the number one reason why motors fail is through winding breakdown, closely followed by bearing failures, he believes that all too often, customers accept these failures without understanding the root cause.

“In our experience, we believe that many of these winding and bearing failures are a direct result of motors running too hot,” says Johnny.

“For example, you may be told that your bearing has run dry. While in some instances this may be down to a poor re-greasing regime, it is also possible that the motor was simply too hot and the grease degraded prematurely.”

While efficiency classifications, as defined by Eff1, Eff2 and Eff3, have helped customers recognise the difference between a poor energy efficient motor and a high energy efficient one, Dolin is concerned that the classification is also being taken as a measure of the reliability of a machine.

Today Eff1 is perceived as high quality. The belief is that you pay a higher price for a higher efficiency motor and that the certification also indicates high quality and high reliability.

“This is just not the case,” argues Johnny. “Our experience shows that there are some motors which achieve Eff1 status at the cost of significant drawbacks. These manifest themselves in many ways: increased running temperatures and excessive noise being a couple of examples.”

Johnny proposes that “reliability” could be defined as the sum of efficiency plus temperature rise. He argues that each of these elements directly affect each other and therefore, the quality and reliability of a motor.

Eff1 is easy to achieve in all but the smallest frame sizes by simply increasing the amount of active material in the motor – more copper in the slots and smaller air gaps in the design.

Yet the challenge is that the IEC34-2 sets tolerances for efficiency which are quite wide. Dolin fears some manufacturers are declaring efficiencies that are at the uppermost of the tolerance band, while delivering motors close to the lower tolerance level, or at worse outside this band.

Meanwhile, the temperature rise of a motor can also be higher than you would expect from an Eff1 motor. “Hold a thermal camera up to the motor and you can see how hot it is running.”

In the case of the windings, higher temperatures degrade the winding insulation more rapidly. While for the bearing end shields, higher temperatures can lead to premature degradation of the grease and increased re-greasing intervals.

“Clearly, the cooler the running temperature the better,” says Johnny. “Lowering the temperature by just 10 to 15 degrees C can double the life of the winding yet most catalogues do not give this specific information.”

Dolin has observed that some cheaper motors are generating more heat. The normal frame surface temperature in a high reliability motor running at full load can be as low as 60 to 80 degrees C.

“Yet lower reliability motors often run in excess of 90 degree C and have even been recorded at well over 100 degree C.”

Temperature affects the re-greasing intervals of bearings. Dolin assumes that bearings will run at 80 degree C based upon an assumed an ambient of 25C.

Should the bearing temperature increase by 15 degree C, then the re-greasing interval should be halved. If the temperature decreases by 15 degree C, the interval can be doubled.

“The problem is that this data is not always highly visible, and so many engineers are probably not aware of this fact” says Johnny. “As a result if a catalogue states 10,000 hours and the temperature increases by 15 degree C, then the bearing would need re-greasing in 5,000 hours. But how is a user meant to know this?

End-users with continuous process applications should ask their supplier to provide the winding and bearing temperature criteria from the type test reports.”

Temperature can lead to problems in other areas. More active materials usually mean more heat being generated. To keep the motor within stated temperature limits, larger fans are employed to provide more cooling air. Larger fans mean more noise.

“Remember, a 3dB increase in noise level is equivalent to a doubling of the audible noise of the motor,” says Johnny. “So if, say, a 200kW motor is showing a 77dB against one showing a 70dB rating, then the 7dB increase in noise equates to the motor being about four times noisier. This should set alarm bells ringing. Higher noise levels could mean that the temperature is higher which affects the overall motor reliability.”

Dolin say that getting the right balance between efficiency, temperature rise and noise will go a long way to lower life cycle costs, lowering running costs and increasing the overall reliability of an electric motor.

“If you use motors in a 24/7 continuous process, then the last thing you need is a hot motor, which eventually fails. The cost to production can be immense. We believe that these users should have access to temperature rise information and should understand the importance of the noise levels. This would help them choose a motor with greater reliability.”

Johnny and his team at Dolin are presently gathering statistics and data and are keen to hear from any end-user who believes that a motor has failed prematurely through temperature issues or from users interested in learning ways to identify a “reliable” motor.